Flexible circuitry promises a host of innovative biomedical, security, wearable and other products. To date, flexible circuits have offered only limited performance because plastic substrates aren’t compatible with the high temperatures/harsh processes needed to make high-performance CMOS devices.
Some attempts have been made to fabricate high-performance CMOS on silicon substrates and then transfer the devices to plastic, but this has been complex and expensive. At the International Electron Devices Meeting (IEDM), for the first time, a way around this will be unveiled. IBM researchers will demonstrate high-performance state-of-the-art CMOS circuits —including SRAM memory and ring oscillators—on a flexible plastic substrate. The image above is a photo of the final 100-mm-diameter flexible ETSOI circuit on plastic.
The extremely thin silicon on insulator (ETSOI) devices had a body thickness of just 60 angstroms. IBM built them on silicon and then used a simple, low-cost room-temperature process called controlled spalling, which essentially flakes off the Si substrate. Then they transferred them to flexible plastic tape.
The devices had gate lengths of <30 nm and gate pitch of 100 nm. The ring oscillators had a stage delay of just 16 ps at 0.9 V, believed to be the best reported performance for a flexible circuit. A slight degradation of delay for the flexible sample after the layer transfer comes from degradation of p-FET performance due to strain effects.
The image below is a cross-sectional view taken by a TEM electron microscope after selective removal of the residual silicon, confirming the structural integrity of the device.